1. Field of the Invention
This invention relates to retrievable well tools used in subterranean oil and gas wells under extreme conditions of temperature and pressure and more particularly to packing element systems employed on well tools, such as retrievable packers, bridge plugs and gravel packing tools.
2. Description of the Prior Art
Downhole well tools, employed in subterranean oil and gas wells are normally intended either for permanent installation within the well bore or are of the retrievable type which may be inserted into the well bore and subsequently removed. For example, downhole packers commonly used to establish a seal in the annulus between the well casing and a smaller diameter production tubing string inserted into the casing can be intended either for permanent installation or for subsequent retrieval. Permanent well packers can be set at a desired location within the well bore by means of mechanical tubing or wireline manipulation or by the use of hydraulic or hydrostatic pressure to set the permanent packer. Retrievable packers can also be set by hydraulic or mechanical manipulation. Retrievable packers can also be released by either mechanical or hydraulic manipulation. For example, retrievable packers are commonly released by manipulation of a retrieving tool inserted into the bore of the retrievable packer on a tubing extending to the surface of the well.
Once a permanent packer has been set at the prescribed location within the well, it can only be removed by milling or drilling the packer, thus destroying the packer. A conventional permanent packer cannot be returned to the surface of the well in substantially one piece to be redressed for further use. Retrievable packers can be redressed after retrieval and are suitable for further use.
In general, permanent packers are suitable for use at higher temperatures and pressures than comparable retrievable packers. One reason for the higher temperature and pressure ratings which can be achieved with permanent packers is that permanent packers have hitherto been designed with a greater capability for resisting extrusion of the packing element. Both permanent and retrievable packers are normally inserted with a well bore with adequate clearance between the packer and the well bore to avoid interference as the packer is run into the well. When the packer is set, radially expandable slips are actuated and moved into engagement with the well casing. An annular seal or packing element commonly fabricated of a resilient or elastomeric material is expanded into engagement of the well casing in response to axial compression exerted on the packing element. The clearance between the housing of the packer or well tool and the well casing provides an annular area into which the packing element, subjected to axial compression, can extrude.
In permanent packers, bridging or extrusion preventing rings formed of a malleable metallic material, are commonly employed to prevent extrusion into the area between the packer housing and the well casing. These metallic extrusion preventing rings are expandable into engagement in the casing upon the application of an axially compressive force sufficient to expand the packing element into sealing engagement with the casing. These extrusion preventing rings effectively seal off all, or a portion of, the annular clearance area and are of a sufficient strength to withstand both extreme pressures applied to the packer and to prevent extrusion of the packing material subjected to extreme temperatures. Outward expansion of the extrusion preventing rings brings them into engagement with the casing and prevents subsequent removal of the packer unless the rings can be retracted. Some extrusion preventing rings are plastically deformed when the packer is set. These plastically deformed extrusion preventing rings thus lack sufficient elastic memory to retract from engagement with the casing when axially compressive loads are removed.
Even if the extrusion preventing rings retain inherent elasticity, retraction of the rings is prevented if the packing element has been permanently deformed. Such permanent deformation can occur when the resilient material comprising the packing element has taken on a permanent set upon being subjected to elevated temperature and pressure for a certain period of time. The packing element can thus wedge the extrusion preventing means in such a manner that retraction of even an elastic extrusion ring is prevented by engagement with the permanently set sealing element. The extrusion preventing rings can also be permanently wedged into engagement with the casing when the structure of the packing element has been deformed by fracture under elevated temperatures and pressure. For example, the radial expansion of extrusion preventing rings often leaves a circumferential gap between adjacent segments or ends of the extrusion preventing ring. When subjected to elevated temperatures and pressures, particularly for a sustained period of time, the material forming the packing element will extrude through these circumferential gaps. Extrusion through these gaps will be accompanied by a destruction of the molecular bonds of the resilient material forming the packing element, thus destroying the elastic memory of the packing element material. With the packing element material thus wedged in circumferential gaps separating adjacent elements of the extrusion preventing rings, these circular gaps cannot be closed and the extrusion preventing rings subsequently cannot be retracted out of engagement with the casing. Thus a packer employing extrusion preventing rings of the type formerly used on permanent packers could not be released from engagement with the well casing even if the anchoring slips holding the packer in place could be disengaged.
Only those retrievable packers employing extrusion rings capable of withstanding only small amounts of shear applied upon longitudinal movement of the packer body could heretofore be used with retrievable packers. For example, retrievable packers having plate-like extrusion preventing rings or shoes could be employed, since the relatively thin plates forming the extrusion barriers or shoes would be deformed or bent out of engagement with the casing upon application of sufficient force to the packer housing to cause the packer to move relative to the well casing. Of course, such relatively weak extrusion barriers or shoes could not withstand extreme temperature and pressure forces which can be encountered under certain conditions. Thus the normal practice is to use a permanent packer when extreme conditions in temperature and pressure are anticipated, especially when the packer is intended to be used for a protracted period of time.
U.S. Pat. No. 4,326,588 discloses one permanent packer intended for use under extreme conditions of temperature and pressure. The permanent packer disclosed therein employed a primary sealing element fabricated from a material such as polytetrafluoroethylene which has good chemical resistance. Wire mesh elements are disposed on opposite sides of the centrally located main sealing element to prevent extrusion of the main sealing element at elevated temperature and pressure. Radially expandable extrusion barrier rings of the type commonly employed on conventional permanent packers are also employed to further resist extrusion. U.S. Pat. No. 4,326,588 does not, however, provide means for retracting the packer from the well casing to permit retrieval. A retrievable packer capable of withstanding elevated temperature and pressure conditions, heretofore requiring the use of a permanent packer, and capable of disengagement from the well casing and retrieval is disclosed and claimed herein.